• Energy Research

Abstract In this study, a parabolic dish concentrator with two types of cavity receivers was investigated. The designed experimental setup included a parabolic dish concentrator, cavity receivers, a heat exchanger system, and hydraulic circuit unit. Two optimized shapes of cubical and cylindrical cavity receiver were made and studied. Also, numerical modeling was developed for predicting the cavity receiver performance. The numerical results of the cavity receivers show a good agreement with the experimental results. The results indicated that the receiver heat gain and thermal efficiency of the cavity receivers had a similar trend compared to the temperature difference of the heat transfer fluid between the inlet and outlet of the cavity receivers. The results also clarified that the thermal efficiency of the cubical cavity receiver was higher than the thermal efficiency of the cylindrical cavity receiver in the steady-state period. The average thermal efficiency of the cubical and cylindrical cavity receiver was obtained as 65.14% and 56.44% in the steady-state period, respectively. The cubical cavity receiver can be recommended for an efficient heat gain, in comparison with the cylindrical cavity receiver based on the conducted experimental tests on 11 October, and 26 September 2016.

Abstract In this research paper, a hybrid solar desalination system has been employed. The hybrid solar desalination system includes photovoltaic thermal panels, solar dish concentrator, and humidification-dehumidification desalination unit. The humidification-dehumidification desalination unit comprises a closed-air open-water flow configuration, and the solar dish concentrators are utilized for water heating. Examination of three different shapes of cavity receiver including cylindrical, cubical and hemispherical, as the solar dish absorbers, was carried out. Thermal oil was considered as the solar working fluid. The absorbed solar heat was transferred to the desalination unit using a heat exchanger. In the hybrid solar desalination, photovoltaic panels were used to generate the required power. Water flow was considered at the back of the photovoltaic panels for preheating and improving the photovoltaic efficiency. The principal aim of the current study is to propose hybrid solar desalination system to generate power, and produce freshwater. The solar desalination's performance was examined in terms of various solar dish parameters and different humidification-dehumidification desalination parameters. Examination of various solar dish parameters, including the solar working fluid's inlet temperature and the cavity shapes, was carried out. Also, some humidification-dehumidification desalination parameters, including the water to air flow ratio and the water flow rate, were considered. The effects of these four parameters were investigated on the water production and the gain output ratio. Based on the results, it was found that there was an increase in the production of freshwater by raising the water flow rate, decreasing the solar working fluid inlet temperature and increasing the air flow rate. Besides, there was an increase in the gain output ratio by increasing the water flow rate, increasing the inlet temperature, and increasing the air flow rate. Finally, the highest freshwater production and lowest gain output ratio were resulted by the hemispherical cavity receiver.

Abstract In this study, a solar dish collector is considered with a rectangular cavity receiver. The investigated parameters were included the receiver aperture area (a 2 ), receiver tube diameter, cavity depth, inlet temperature and mass flow rate through the receiver. This cavity receiver is used as the heat source of the organic Rankine cycle (ORC). The ORC system is considered with R141b as the working fluid at the saturated condition. The main objective is the calculation of the optimum parameters for attaining the maximum overall thermal efficiency of the system. With the help of the receiver modeling techniques, the optimum aspect ratios of 54.25, 40.69, 32.56, 23.25 and 18.09 are identified for five cavity depths of 0.5a, 0.75a, 1a, 1.5a, and 2a where the accompanying optical efficiencies are 89%, 92%, 94%, 96% and 96%, respectively. It is concluded that for increasing the collector efficiency and overall thermal efficiency, a higher mass flow rate, and a lower inlet thermal oil inlet temperature are required. Also the results show that the optimum characteristic of the cavity for achieving highest collector efficiency and the highest overall thermal efficiency includes smaller tube diameter, lower inlet temperature of the working fluid, and the cavity depth of 1a.

In this study, the thermal performance of a parabolic dish concentrator with a rectangular-tubular cavity receiver was investigated. The thermal oil was used as the working fluid in the solar collector system. The performance of the cavity receiver was studied in two ways as a numerical modeling method and the artificial neural networks (ANNs) methodology. In this study, three variable parameters including the different tube diameters equal to 5, 10, 22, and 35 mm, and different cavity depths equal to 0.5a , 0.75a , 1a , 1.5a , and 2a were considered. The purpose of this study is the prediction of the thermal performance of the cavity receiver in different amounts of solar irradiance, the cavity depth, and the diameter of tube by the ANN methodology. The main benefit of the ANN method, in comparison with the numerical modeling method, is the calculation time and cost saving. The results reveal that the ANN method can accurately predict the thermal performance of the cavity receiver at different variable parameters of the cavity depth, and tube diameter with R 2 = 0.99 for each prediction.

AbstractIn this research, the small segments parabolic dish concentrator (SSPDC) with a modified cavity was optically and thermally investigated. The SSPDC consisted of small mirrors that were tracking the sun individually. The SSPDC allows using different concentration ratios and aperture areas. Oil, pure water, and water + perypolen glycol (PG) were evaluated as working fluids. Different structural characteristics of the solar system were investigated, including dish focal distance, cavity outer diameter, and cavity aperture area. Then the investigated solar system was explored as a heat source of an organic Rankine cycle (ORC) unit for providing the power for a house. Finally, some environmental parameters of the investigated ORC units, such as carbon dioxide emission and carbon dioxide credits, were evaluated. The results reveal that the investigated solar unit had a dish depth of 0.14 m, a focal distance of 1 m, a cavity aperture outer diameter of 0.1 m, and a cavity aperture inner diameter of 0.08 m for a 10‐mrad optical error and a 1° tracking error. Usage of pure water is recommended for low‐temperature application, water + PG for medium‐temperature application, and oil for high‐temperature application. It was concluded that two units of the proposed ORC could provide energy to the house with solar radiation of more than 800 W/m2 and the number of units should be increased for achieving the required power with decreasing solar radiation.

Abstract In this research, an energy harvesting system was developed for power generation using a combination of an Archimedes Screw Turbine (AST) and a solar Organic Rankine Cycle (ORC) system. An AST was numerically used and optimized for producing mechanical power as an energy harvesting technique. Different structural parameters including the screw inclination angle, number of flights and the screw length were considered. A parabolic trough concentrator was numerically modeled as a heat source of the ORC system. Two different types of absorber were considered using a smooth and corrugated tube. Different ORC working fluids were investigated in the solar ORC system including R134a, R245ca, R245fa, R152a, R113, R11, and R114b. The results of numerical modeling were validated with experimental results and good agreement was found. The results revealed that R113 at the saturated condition at turbine inlet gave the highest ORC net power, ORC efficiency, and total efficiency compared to the other investigated working fluids. The solar PTC system with the corrugated tube showed the higher ORC net power, and overall efficiency compared to the smooth tube as the PTC receiver. The highest efficiency resulted in the screw length of 1.5 m was 58.24% with inclination angle of 25° and flight number of 1. Finally, the optimized characteristics of power generation system including a solar ORC system and a screw turbine (hybrid system) were presented to harvest energy. Application of the presented hybrid system is an acceptable way for increasing and optimizing the ORC power generation.

Abstract The application of nanofluids is accounted as an effective way for improving the thermal performance of solar systems. In the current study, MWCNT/thermal oil nanofluid was experimentally tested as a solar heat transfer fluid. A dish collector, using a wounded cylindrical cavity receiver, was considered as the solar system. The main aim of this work was the experimental investigation of the cylindrical cavity’s performance, using MWCNT/thermal oil nanofluid. Two experimental models were suggested for the prediction of the cavity’s thermal efficiency, by considering MWCNT/thermal oil nanofluid and pure thermal oil in the steady-state period. It was indicated that the heat loss coefficient of the cylindrical cavity receiver decreased by the application of nanofluid. The results revealed the average thermal efficiencies of 63.9% and 56.44% for MWCNT/thermal oil nanofluid and pure thermal oil, respectively. The thermal efficiency was changed form 60.04% to 64.76%, by using the nanofluid, in the steady-state period. Also, it was observed that the thermal performance of the cavity receiver was improved by 13.12%, by using MWCNT/thermal oil nanofluid. Based on the obtained results, the application of MWCNT/oil nanofluid is recommended in the solar dish concentrators with cavity receiver.

Abstract Organic Rankine Cycle (ORC) is a promising electricity production technology that exploits low and medium heat sources. Usually, renewable and alternative heat sources can be used in order to feed an ORC with heat. The exploitation of geothermal energy is a usual and sustainable way to feed an ORC because it is a sustainable, abundant, economical and environmentally friendly choice. The main objective of this study is to review and to discuss the geothermal-driven ORC systems for power generation in a detailed way. Moreover, the special novelty is the emphasis that is given in the use of geothermal ORC systems inside cogeneration, trigeneration and polygeneration units. Both experimental and numerical investigations are included in the present work, while they are discussed in energy, exergy and economic terms. It is found that the geothermal-driven ORC systems are viable investments with relatively low payback periods, as well as these systems lead to high energy efficiency. Moreover, it is concluded that a 20% to 30% increase in the performance of geothermal-fed ORC systems is possible by optimization. Lastly, it is useful to state that the polygeneration systems that include geothermal-driven ORCs are promising units that present high exergy efficiency values.

Abstract The objective of this work is to compare two cavity receivers for a solar dish concentrator. The spiral and the conical cavities are investigated using a developed thermal model. The analysis is optical, thermal and exergetic for different operating temperatures and flow rates. The developed thermal model is combined with an optical tool in order to simulate properly the solar dish collector and it is validated for the case of the spiral absorber with experimental results. Every receiver is separated in the different coil and every coil is simulated separately in order to increase the model accuracy. Totally, 13 coils are used for the spiral design and 11 for the conical design. The location of the receiver in every case is optimized in order to achieve maximum optical efficiency. The results show that the conical design leads to a 1.38% increase in the optical efficiency due to the increased intercept factor. The thermal efficiency enhancement with the use of conical design is found to be 5.63% at 100 °C and 40.45% at 200 °C, while the exergy efficiency enhancement 6.67% at 100 °C and 42.06% at 200 °C.